Such friction ring assemblies are used, in particular, for downhole drill motors where the shaft is the shaft of a turbine and the outer body is the fixed tubular body of the turbine. The said first subassembly then includes an axial succession of friction rings, each of which may have two friction surfaces, one on each of two opposite faces, with each friction surface co-operating with the friction ring of a respective second subassembly which is resiliently biased from a respective frame.
The term "turbodrill" may be applied to such a turbine motor when equipped with a suitable tool. Turbodrills need to be equipped with abutment surfaces for transmitting large longitudinal forces between the rotating shaft and the outer body. The friction rings are made of steel and the friction surfaces are treated to have suitable surface hardness and a low coefficient of friction. They may, for example, be constituted by tungsten carbide, aggregations of diamonds, or hard metal alloy. The resilient axial thrust members serve to distribute the load over the various friction rings, and to damp axial shock and vibration.
It is easier to manufacture a rigid single part friction ring than it is to manufacture a plurality of independent friction tabs which have sometimes been used to replace a friction ring in the second subassemblies.
One such prior art friction ring assembly is described in French patent publication No. 2 157 206 (Alsthom), and also in the corresponding U.S. Pat. No. 3,858,668 (Jean Bell).
In this prior art assembly, the friction rings of the first subassemblies, ie. the rings which rotate with the shaft, have their rear (ie. non-friction) surfaces mounted on a layer of elastomer which constitutes the said resilient thrust member. The rings are prevented from rotating relative to the shaft by the adherence of the said elastomer layer or by being keyed to the shaft, and any axial displacement is accompanied by high levels of friction.
This prior art assembly has considerably increased the service life of turbodrills, but wear on the friction surfaces still limits the service life excessively. This wear is greatly increased when the surrounding medium is a drilling mud transporting abrasive particles.
Preferred embodiments of the present invention provide an oscillating friction ring assembly for transmitting resilient axial thrust to a shaft while increasing service life, particularly in the presence of drilling mud. Such embodiments are capable of transmitting large axial forces and are simple to manufacture.